Since the start of 2017, 76 fishing vessels have ended up on Japanese shores. Some of the stranded vessels have been labelled "ghost ships" because of the skeletons found on-board.

Dead bodies have been discovered on at least five ships in the past month. Eight skeletons were found in the hull of a vessel that washed ashore on Miyazawa beach in northern Japan on 27 November.

The Japanese coast guard has tightened patrols in recent weeks amid fears that North Korean spies are sheltering on the stranded ships.

"The government is well aware that this is causing great anxiety to local people," Japan's Chief Cabinet Secretary Yoshihide Suga said after a ship bearing a North Korean military emblem landed on a Japanese island in late November.

"The police and coast guard... are working to clarify the situation," Suga said. "Once we have the facts, we will respond firmly."

Over the summer, the Japanese coast guard issued 820 warnings to North Korean ships that sailed into the Yamato Bank fishing grounds within Japan's economic zone, the Financial Times reported.

Professor Satoru Miyamoto, from Seigakuin University, told news.com.au that there was no reason for concern and said the rising number of ships was likely due to an increasing demand for fish.

"The number of wealthy people is growing and they're seeking a healthy diet. Fishing is meeting that demand," he said.

He said the increase highlights Kim Jong-un's decision to "expand the fisheries industry as a way of increasing revenue for the military. They are using old boats manned by the military, by people who have no knowledge about fishing. It will continue."